U.S. Pat. No. 3,984,733 which issued in the name of the present inventor discloses an arc proof dual interlock safety switch which is intended to fulfill the same purpose as the switch assembly of the present invention. The interlock switch described in the patent includes two microswitches connected in series, and which are actuated when the access door is closed, this being accomplished by a pair of contact posts mounted on the door which move against a pair of spaced electrically conductive strips, initially to interconnect the strips, and then to move the strips against the actuator buttons of the microswitches. The resulting switch assembly provides electrical switching without arcing when the door is opened or closed.
The assembly of the present invention has certain advantages over the switch described in the patent in that it accomplishes its intended purpose in a simpler and more straight-forward manner. The use of a plug in the place of the contact posts is advantageous in that the posts had a tendency to break, whereas the plug is strong and rigid. Moreover, the plug is capable of more efficient linear travel than the posts, and it applies a more positive force to the electric contacts in the socket. Moreover, the assembly of the present invention obviates the need for the relatively expensive microswitches used in the switch described in the patent.
The interlock switch assembly of the present invention, as is the case with the assembly described in the patent, is constructed principally for use in X-ray systems in order to protect the health and safety of the personnel operating the systems. However, it will become evident as the description proceeds that the switch assemblies have general application for providing an effective electrical interlock which operates in conjunction with access doors, and which positively prevents the application of electric power to equipment when such access doors are open, even in the event of failure of the power relays in the electrical system.
Many types of interlock systems have been used in the past, and even with such prior art systems medical and industrial accidents have occurred because the interlock systems failed, subjecting the operators to exposure of X-rays. An important feature of the interlock assembly of the present invention, as is the case with the assembly of the patent, is that it assures that the power is cut off from the X-ray machine when the access door is opened, regardless of any faults that may have occurred in the energizing circuit. The circuitry associated with the interlock switch assembly of the invention is such that power can be applied to the X-ray unit only when the door is closed, at which time the circuits are closed mechanically and electrically, and only then can the unit be activated. Arcing is prevented in the use of the interlock switch of the invention, since the switch serves to open the circuit to the power control relay in the energizing circuit to de-energize that relay prior to the physical breaking of contact with the power source itself.